Bag House. Cleaning Sequences Intermittently cleaned baghouses consist of a number of compartments or sections. One compartment at a time is removed from

Bag House

Cleaning Sequences• Intermittently cleaned baghouses consist of a number of

compartments or sections. One compartment at a time is removed from service and cleaned on a regular rotational basis. The dirty gas stream is diverted from the compartment being cleaned to the other compartments in the baghouse, so it is not necessary to shut down the process.

• Continuously cleaned baghouses are fully automatic and can constantly remain on-line for filtering. The filtering process is momentarily interrupted by a blast of compressed air that cleans the bag, called pulse-jet cleaning. In continuous cleaning, a row of bags is always being cleaned somewhere in the baghouse. The advantage of continuous cleaning is that it is not necessary to take the baghouse or a compartment out of service for bag cleaning.

Source: USEPA, APTI Course SI: 412A, Fabric Filter Operation Review

Types of Bag Cleaning

• shaking, • reverse air, • pulse jet, • sonic


Shaking Mechanical shaking is accomplished by using a motor that drives a shaft to move a rod connected to the bags. It is a low energy process that gently shakes the bags to remove deposited particles. The shaking motion and speed depends on the vendor’s design and the composition of dust deposited on the bag. The shaking motion is generallyin the horizontal direction.


Shaking should not be used when collecting sticky dusts. The force needed to remove sticky dust can tear or rip the bag.


Shaking



Reverse Air

Reverse-air cleaning baghouses are compartmentalized to permit a section to be off-line for cleaning. In a reverse-air baghouse, the flow of dirty gas into the compartment is stopped and the compartment is backwashed with a low pressure flow of air. Dust is removed by merely allowing the bags to collapse, thus causing the dust cake to break and fall into the hopper.

During the filtering mode, the compartment’s outlet gas damper and inlet gas damper are both open. When bag cleaning begins, the outlet damper is closed to block the flow of gas. The bags are allowed to relax for a short time and the reverse air damper located at the top of the compartment is opened to bring reverse air for bag cleaning into the compartment. The reverse air flow usually lasts from about 30 seconds to as long as several minutes. During this time, dust falls into the hopper. Reverse-air baghouses also have by-pass dampers that allow the dirty gas to by-pass the compartments during malfunctions and start up periods.

The cleaning action is very gentle, allowing the use of less abrasion resistant fabrics such as fiberglass.

Reverse-air cleaning is generally used for cleaning woven fabrics. Cleaning frequency aries from 30 minutes to several hours, depending on the inlet dust concentration and the pressure drop of the baghouse. The cleaning duration is approximately 10 to 30 seconds; the total time is 1 to 2 minutes including time for valve opening and closing, and dust settling.

Reverse-air baghouses use very large bags (as compared to shaker or pulse-jet baghouses) ranging from 8 to 18 inches in diameter and from 20 to 40 feet in length.


Pulse Jet


The most commonly used cleaning method is the pulse-jet or pressure-jet cleaning. Baghouses using pulse-jet cleaning make up approximately 40 to 50% of the new baghouse installations in the U.S. today.

The pulse-jet cleaning mechanism uses a high pressure jetof air (compressed air-induced pulse) to remove the dust from the bag. Bags in the bag-house compartment are supported internally by rings or metal cages. Bags are held firmly in place at the top by clasps and usually have an enclosed bottom (the bag is sewn closed at the bottom). In another design, a snap ring is sewn into the top of the bag which fits into the tube sheet opening. The cage slides inside the bag and the top of the cage sits on the tube sheet. Dust-laden gas is filtered through the bag, depositing dust on the outside surface of the bag. Pulse-jet cleaning is used for cleaning bags in an exteriorfiltration system.



Pulse-jet units are usually operated in a “non-dust cake” mode. Bags are pulsed frequently to prevent the formation of a thick cake and to keep the unit from having a high pressure drop across the dust cake and felted filter. However, sometimes a dust cake is desired in cases where woven bags are used in a pulse-jet baghouse.

The bag cleaning by the pulse occurs in approximately 0.3 to 0.5 seconds. The pressures involved are commonly between 60 and 100 psig (414 kPa and 689 kPa). Some vendors have developed systems to use a lower pressure pulsing air (40 psi).

Most pulse-jet baghouses use bag tubes that are 4 to 6 in. (10.2 to 15.2 cm) in diameter. The length of the bag is usually around 10 to 12 ft (3.05 to 3.66 m), but can be as long as 20 ft (6.1 m). The shaker and reverse-air baghouses use larger bags than the pulse-jet units.



Sonic In a few systems, shaking is accomplished by sonic vibration. A sound generator is used to produce a low frequency sound that causes the bags to vibrate. The noise level produced by the generator is barely discernible outside the baghouse. Sonic cleaning is generally used along with one of the other cleaning techniques to help thoroughly cleandirty bags.


Baghouses are designed by considering a number of variables: pressure drop, filter drag, air-to-cloth ratio, collection efficiency


The total pressure drop equals the pressure drop across the filter plus the pressure drop across the cake and is given as:

Where:Δpf = pressure drop across the clean fabric, in. H2O (cmH2O)k1 = fabric resistance, in. H2O/(ft/min) [cm H2O/(cm/sec)]vf = filtration velocity, ft/min (cm/sec)

Δpc = pressure drop across the cake, in. H2O (cmH2O)k2 = resistance of the cake, in. H2O/(lb/ft2-ft/min) [cm H2O/(g/cm2-cm/sec)]ci = dust concentration loading, lb/ft3 (g/cm3)vf = filtration velocity, ft/min (cm/sec)t = filtration time, min (sec)


Total pressure drop

Filter DragFilter drag is the filter resistance across the fabric-dust layer. The equation for filter drag essentially gives the pressure drop occurring per unit velocity. It is a function of the quantity of dust accumulated on the fabric and is given as:

Where: S = filter drag, in. H2O/(ft/min) [cm H2O/(cm/sec)]Δp = pressure drop across the fabric and dust cake, in. H2O (cmH2O)vf = filtration velocity, ft/min (cm/sec)




Overall pressure drop of a multi-compartment baghouse

Performance curve of a pulse-jet bag or a row of bags


Filtration Velocity: Air-To-Cloth RatioThe terms filtration velocity and air-to-cloth (A/C) ratio can be used interchangeably. The formula used to express filtration velocity is:

Where: vf = filtration velocity, ft/min (cm/sec)Q = volumetric air flow rate, ft3/min (cm3/sec)Ac = area of cloth filter, ft2 (cm2)


The term gross air-to-cloth ratio refers to the total amount of cloth area used to filter the entire flue gas stream.

The term net air-to-cloth ratio is used to describe the net amount of cloth available for filtering when one or more baghouse compartment is taken off-line for maintenance or bag cleaning.


Collection Efficiency

Extremely small particles (less than 1 μm in diameter) can be efficiently collected in a baghouse. Emission regulations for various industries including municipal waste combustors and hazardous waste incinerators require emission limits of 0.010 gr/dscf. Baghouse units designed with overall collection efficiencies of 99.9% (varying particle sizes) are common.Exhaust air from many baghouses can even be recirculated back into the plant for heating purposes, as long as the gas stream is not toxic.


Filter Construction

Bag filters can be made of woven or nonwoven materials. Nonwoven materials can further be divided as felted or membrane. Most bags are either completely or partially made by weaving since nonwoven fabrics are generally attached to a woven base called a scrim. Woven filters are made of yarn with a definite repeated pattern. Felted(氈、不織布 ) filters are composed of randomly placed fibers compressed into a mat and attached to loosely woven backing material. A membrane(薄膜 ) filter is a special treatment where a thin, porous membrane (expanded polyfluorocarbon) is bonded to the scrim, or support fabric. Woven filters are generally used with low energy cleaning methods such as shaking and reverse-air. Felted fabrics are usually used with higher energy cleaning systems such as pulse-jet cleaning. Membrane filters were developed in efforts to achieve high efficiency particle capture and to handle flue gas conditions where high moisture and resulting high pressure drop problems frequently occur.


Woven Filters


Sieving (on a woven filter)

The true filtering surface for the woven filter is not the bag itself, but the dust layer or filter cake. The bag simply provides the surface for capture of larger particles. Particles are collected by impaction or interception as the open areas in the weave are closed. This processis referred to as sieving. Some particles escape through the filter until the cake is formed. Once the cake builds up, effective filtering will occur until the bag becomes plugged and cleaning is required. At this point, the pressure drop will be exceedingly high and filtering will no longer be cost effective. The effective filtering time will vary from approximately 15 to 20 minutes to as long as a number of hours, depending on the concentration of particulate matter in the gas stream.


Felted FiltersFelted filters are made by needle punching fibers onto a woven backing called a scrim. The fibers are randomly placed as opposed to the definite repeated pattern of the woven filter. The felts are attached to the scrim by chemical, heat, resin, or stitch-bonding methods.To collect fine particles, the felted filters depend to a lesser degree on the initial dust deposits than do woven filters. The felted filters are generally 2 to 3 times thicker thanwoven filters. Each individual randomly oriented fiber acts as a target for particle capture by impaction and interception. Small particles can be collected on the outer surface of the filter.Felted filters are usually used in pulse-jet baghouses. Felted bags should not be used in high humidity situations, especially if the particles are hygroscopic. Clogging or blinding could result in such situations



Flex abrasion resistance : 彎曲磨損抗力


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Bag House. Cleaning Sequences Intermittently cleaned baghouses consist of a number of compartments or sections. One compartment at a time is removed from